1. Field of the Invention
This invention in general relates to a boiler feed pump turbine system, and more particularly to an arrangement for protecting against a loss of signal.
2. Description of the Prior Art
In a steam turbine-generator system, the turbine is supplied with steam from a boiler (which may also be a nuclear steam generator) in accordance with operational needs of the system. In order to supply the boiler with feedwater there is provided a boiler feedwater supply system which includes a boiler feed pump driven by a turbine whose operation is controlled by a boiler feed pump turbine control system. Normally, the boiler feedwater pump requirements are coordinated with the speed and load demands of the main steam turbine-generator system so that in addition to the feed pump turbine control, there is also provided an overall boiler feedwater control system.
In an initial startup mode, the feed pump turbine is brought up to a desired speed by means of an operator using increase and decrease pushbuttons on an operator's panel to adjust a motor driven potentiometer, the output voltage of which is representative of a desired speed reference setpoint. This desired speed is compared with the actual speed of the feed pump turbine and any difference, or error, causes certain steam admission valves to the feed pump turbine to be controlled accordingly. At a certain predetermined minimum speed, control is transferred to the boiler feedwater control system which supplies an analog signal indicative of a certain desired speed of rotation of the turbine (and accordingly of the feed pump).
Typically, below a certain minimum speed, the lower valued one of the signals from either the boiler feedwater control system or the boiler feed pump turbine control system is utilized for comparison with the actual speed signal. As the apparatus reaches the normal operating speed range, the boiler feedwater control system signal takes command and the motor driven potentiometer is caused to provide a maximum output signal so as not to interfere with proper operation. A problem may arise, however, if the boiler feedwater control system signal fails, this condition will be sensed as calling for a minimum speed. Thus, the speed of the feed pump turbine will reduce to a minimum value inconsistent with the actual amount of feedwater flow that is required.
The present invention obviates this objectionable and possibly detrimental mode of operation upon a signal failure.